[clang][DebugInfo] Emit DW_AT_object_pointer on function declarations with explicit `this` (#122928)
In https://github.com/llvm/llvm-project/pull/122897 we started attaching
`DW_AT_object_pointer` to function definitions. This patch does the same
but for function declarations (which we do for implicit object pointers
already).
Fixes https://github.com/llvm/llvm-project/issues/120974